System and method for continuous fabrication of graded structured units using additive manufacturing

ABSTRACT

A system and method of continuous fabrication of multi-material graded structures using additive manufacturing is disclosed. Using multi-material feedstocks and optimized processing parameters, the gradient on composition and structure are controlled to achieve smooth transition from one functional component to another functional component. A multi-material graded structure is produced as the feedstocks are transported from the feedstock reservoir system comprised of many different materials. Interface transition from one functional layer to the next is gradient, controlled by feedstock mixture ratios based on the flow rate control for the feedstock system. Composition includes chemical composition, physical composition, and porosity. Continuous automatic additive manufacturing method makes the fabrication more efficient and avoids joining problems. This method finds application in fabrication of a fuel cell, battery, reformer and other chemical reaction and process units, including structures made of multiple units, such as stacks, that incorporate multiple functional components.

BACKGROUND

There are many different methods to fabricate graded components that have been proposed and developed. Chemical vapor deposition, tape casting, screen printing, slurry-spraying, spray-painting, and slurry coating all have been reported in the literature. Due to the limitations of processing methods, most of these methods are not continuous. Production disruption that occurs when changing the raw material decreases the efficiency of the process, making large production infeasible. Furthermore, most of the composition gradients are reached by two or three layers with different compositions, which makes the gradient still large and discontinuous. There is a lack of a continuous processing method to reach smooth composition graded structure with high production efficiency. It would be useful to overcome these limitations.

SUMMARY

One embodiment disclosed herein is apparatus comprising a multi-material graded structure formed from materials that are mixed and melted into a melt pool utilizing a high energy source.

Another embodiment disclosed herein is method for producing a multi-material graded structure where an additive manufacturing process utilizing a high energy source and multiple material storage is used to produce a melt pool of multi-materials that is formed from materials fed into the melt pool at a precise and predetermined rate and mixture and where the multi-material from the melt pool is deposited upon a substrate to form a solid graded multi-material.

A further embodiment described herein is a method of utilizing a modified job code computer file for Computational Aided Machining in additive manufacturing machines with materials mix ratio description and implementation.

Other embodiments described herein include a system and method for continuous fabrication of graded structured units using additive manufacturing. Multi-material graded structure includes composition gradient, porosity gradient, morphology gradient, grain size gradient, and structural gradient.

According to yet another embodiment, a method is provided for fabrication of a composition gradient fuel cell unit with three components: anode, electrolyte and cathode. Gradient structure is applied at the interface of components. The unit can be fabricated with a laser beam powder feed additive manufacturing machine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates graded structure of units in general composed by functional components with graded interface.

FIG. 2A compares conventional and graded solid oxide fuel cell configuration.

FIG. 2B shows the size of the various graded components for one embodiment of the solid oxide fuel cell configuration.

FIG. 3 illustrates graded structure of battery composed by functional components with graded interface.

FIG. 4 shows a process flow of manufacturing of graded structured unit and components using additive manufacturing.

FIG. 5 shows implantation of one stage of the process illustrating material flow to reach graded structured unit from powder reservoirs to additive manufactured components and units.

FIG. 6 shows traditional information flow in another stage of the process, processing optimization identified in FIG. 4 .

FIG. 7 shows graded structure manufacturing multi-material information flow in stage 300, processing optimization identified in FIG. 1 .

FIG. 8 shows the setup of the laser and powdered material feed used in directed energy deposition (DED) for forming the melt pool on the substrate.

FIG. 9 shows setup of multiple material feed with control to reach multi-material graded structure in solid oxide fuel cell.

FIG. 10A shows a micrograph of a LSM cathode fabricated by additive manufacturing.

FIG. 10B shows a micrograph of a YSZ electrolyte fabricated by additive manufacturing.

FIG. 10C shows a micrograph of a NiO anode fabricated by additive manufacturing.

FIG. 11A shows a bird's eye view and micrographs of YSZ deposited on NiO layers with size of 2.5 cm*2.5 cm.

FIG. 11B shows a portion of the multi-material graded structure in 11A that has been enhanced for viewing of the microstructure.

FIG. 12 is a depiction of the components of a fuel cell and the electrochemical process that produces electricity from a fuel source.

FIG. 13 is a depiction of the multi-material gradient structure showing the gradients between the electrolyte and the anode layer in a fuel cell.

FIG. 14A is a depiction of a fuel cell made with additive manufacturing.

FIG. 14B is a cut away of FIG. 14A showing the internal structure.

DETAILED DESCRIPTION

This disclosure relates to continuous fabrication of graded structure units using additive manufacturing. Using multi-material feeds and optimizing the fabrication processing parameters, the gradient material composition is controlled precisely to produce a smooth transition from one functional material component to another functional material component.

Continuous fabrication of graded structure units using additive manufacturing can be utilized to produce systems and components having tailored properties and structural designs that deliver low cost, high performing products. Energy conversion devices, such as fuel cells and batteries, can be fabricated with graded interfacial structures and in any geometry. This obviates the performance penalties and traditional design constraints that currently exist for electrochemical systems that are fabricated with stacks of flat plates of different materials stacked upon one another with non-graded interfaces. Furthermore, the methods proposed before could not produce free form structure

A compositional and/or morphological and/or structural gradient in component material is preferred in many engineering applications. For example, components with gradient porosity have been applied in medical implants, heat insulation, and thermal shock resistant structures. To join dissimilar materials and decrease the mismatch in properties at interface, functionally graded materials have been developed to achieve unique properties and reach high performance. In fuel cell components, graded materials with porous gradient or chemical composition gradient have been developed and utilized. It was reported that graded structure components in fuel cells reduced interfacial polarization resistances and improved electrochemical performances. Graded structure in anodes, electrolytes and cathodes have all been reported. Another importance of graded structure in fuel cell lies on the requirement of oxygen reduction reaction, which happens at the triple phase boundary, co-incidence of oxygen gas, electronic conductor electrode and ionic conductor electrolyte. Graded structure increases the effective length of the triple phase boundary, improving the overall electrochemical performance.

This described embodiments can replace the conventional methods for fabricating components and systems, such as solid oxide fuel cells (SOFC) and other chemical reactor systems. For more than four decades, SOFCs have been made using conventional methods such as tape casting, calendar rolling, chemical/electrochemical vapor deposition, among others. These methods are costly, time-consuming, require processing in high temperature furnaces, and are constrained to planar or tubular design configurations. And significantly, these conventional methods are not continuous and they do not allow for materially-graded structures. Hence, the interfaces where differing materials meet are the source of performance degradation and subsequent failure. Attempts to overcome these deficiencies have led to lower temperature tolerant materials (600° C.-800° C.) which result in lower overall efficiency. This described embodiments enable fabrication of systems such as SOFCs such that the interfacial properties are satisfactory and sustained high temperature (1,000° C.), high efficiency operation is enabled. The components and parts can be made via a continuous manufacturing process, and geometries for builds are not limited to planes and tubes. The disclosed embodiment find application in, but not limited to, fuel cells, energy storage units, energy conversion units, chemical reformers, and so on.

The embodiments described herein utilize additive manufacturing methods in fabrication. The manufacturing methods include but are not limited to powder additive manufacturing and wire feed additive manufacturing. The methods include but are not limited to powder feed additive manufacturing and powder bed additive manufacturing. The methods include but are not limited to those that employ laser source, electron beam source, and/or kinetic energy source additive manufacturing methods.

One embodiment focuses on fabrication of graded structure. Graded structure includes but not limited to gradients in porosity, chemical composition, architecture, and/or morphology. The general purpose of graded structure is to reach the gradient of properties, including but limited to mechanical, electrical, chemical and magnetic properties.

A graded structure with multiple function layers is illustrated in FIG. 1 . Most units are composed by multiple layers or components. Each layer or component comprises different materials. The interface of layers usually has sharp contrast of property. For example, thermal shock may be introduced due to difference in thermal expansion coefficients. A graded structure is introduced at the interface to reduce the sharp contrast. As shown in FIG. 1 , the interfaces or boundaries are blurry. By introducing the gradient structure at the interface transition in fuel cells, the change in thermal expansion coefficient and mechanical properties are gradient. The life of the SOFC will be extended due to the reduced thermal induced stress during operation cycles. Furthermore, the efficiency of the fuel cell unit will be increased by increasing the interface area between the electrode and electrolyte. The interfaces of layers in FIG. 1 is linear or planar. The present embodiments are not limited to planar or tube shaped interface. Due to the flexibility of additive manufacturing method, there is no limitation to the shape of interface.

The capability to customize electrode, electrolyte, and interconnecting functional elements can lead to increased operating efficiency, decreased performance degradation rates, and increased number of on/off cycles. Overall system efficiency can be defined as the electrical power delivered divided by the lower heating value of fuel consumed. Additional efficiency gains can be accrued if waste heat is productively utilized. On/off cycles induce thermal stresses due to thermal expansion rate differences between the electrodes, electrolytes, and interconnect. Graded interfaces are expected to lessen the thermal stresses during on/off cycles.

One example of the graded structure is applied in a solid oxide fuel cell, as illustrated in FIG. 2A. The left figure shows a conventional configuration of a solid oxide fuel cell. Components of different functional layers are stacked together. The layers include but not limited to interconnect, cathode, electrolyte, and anode. The layers are not limited to planar or tube configuration. The stacks are connected by, but not limited, physical contact, deposition, diffusion, joining, etc. In a conventional configuration, the boundaries between the functional layers are clear. The right figure shows a graded configuration of solid oxide fuel cell in accordance with embodiments described herein. Interfaces of the functional layers are blurry. One non-limiting example is a structure in which the electrolyte comprises yttria stabilized zirconia (YSZ), and the anode comprises nickel reduced from nickel oxide. The interface between the YSZ and nickel oxide comprises multiple very thin layers smoothly changing the chemical composition with a small gradient.

A solid oxide fuel cell has two categories: anode support or cathode support. The size of the unit cell is in the range of 4-2500 cm² with side 5-50 cm. The thickness of the electrolyte is between 3-300 μm. The gradient change from electrode to electrolyte is between 10 μm⁻¹-10 mm⁻¹. In other words, 20% composition change within 4-80 μm. Several different embodiments for SOFC structures can be considered, depending upon desired geometry (planar, tubular, monolithic, unique geometry) of final product. In one tubular embodiment, the cathode is relatively thick compared to the electrolyte, anode, and interconnect. For this case, a graded structure may be envisioned as shown in FIG. 2B. The grading between cathode and electrolyte may be uniform at 10% grading per 200 μm thickness. Similarly, the anode in such embodiment may be graded at a higher level, at 10% grading per 15 μm thickness. This is shown in FIG. 2B where a graded SOFC configuration made by directed energy deposition (DED) (10% grading per 200 μm) is shown.

Alternative grading scales may also be implemented. For example, the cathode in FIG. 2B may be comprised of pure, bulk cathode material at its interior, and the grading may occur only near the interfaces. In this case, the cathode-electrolyte structure may be graded at 10% per 10 μm thickness, assuming only the 100 μm portion of the cathode, closest to the electrolyte, is graded. Other embodiments may be fabricated with electrolyte, anode, or interconnect comprising the thickest component.

A similar approach is applied to other devices, such as battery, as illustrated in FIG. 3 . Functional layers in a battery include, but are not limited to, positive grid, positive plate, separator, negative grid and plate. The interfaces between the functional layers are designed to be transient with graded structure. The present embodiment is not limited to fuel cells, battery, chemical transformer.

FIG. 4 shows the process flow of system in present embodiment. It includes but not limited to three steps: product design, materials design and process optimization. The overall process is designated as 96. In stage 100, product design, the CAD file with geometry information will be created to satisfy the function of parts. In stage 200, materials design, the material composition will be decided to reach the mechanical, electrical, and chemical requirement for parts. Gradient of chemical composition is also decided in this stage. In stage 300, additive manufacturing, the processing parameters and procedures will be optimized to achieve desired microstructure and superior properties.

The detailed description of implantation of materials design, including material flow in stage 200 is illustrated in FIG. 5 . The powders are stored in a series of reservoirs, 201, 202, . . . . Composition is controlled by the ratio of powder mixed from the reservoirs. Smooth transition between the functional layers is reached by controlling the flow rate of the multiple material pool. Flow of each material pool is controlled by its own flow rate control 211, 212, . . . . The mixed powder is mixed and carried to powder tube 221. Then feed to additive manufacturing machine 300. The mixed powder from multi-material powder reservoirs are fed to nozzle in powder feed additive manufacturing machine or powder delivery system in powder bed additive manufacturing machine.

Traditional data flow in additive manufacturing is illustrated in FIG. 6 . CAD file 160 obtained from stage 100, product design, are converted to slice files 162. Then the slice files are transferred to machine CAM file in Job code 164. These file can be, for example, in the format of G-code or M-code, depending on the machine and terminology. The job code is uploaded or transferred to an additive manufacturing machine control panel 166 to initiate the building processing.

In contrast, the data flow in present embodiments disclosed herein has more controls and selections, as shown in FIG. 7 . CAD files 101 from stage 100 are fed into slicing tools to generate slice files 302. With powder composition input files 303, generated from stage 200 in material design, the slice files are transferred to G-code or Job code 304. The G-code in the present embodiment has more information in the material selection and powder feed rate ratio for multiple powder reservoirs. Guided by optimized processing parameters 306, the part with transient interface are produced by additive manufacturing processing 305 based on the G-code 304. The slicers or slicing tools may also be coupled with a machine learning algorithm to optimize gradient generation and produce a generative design.

The embodiments described herein may utilize a laser-based (DED) additive manufacturing process to continuously fabricate multi-material graded structures in conventional and novel geometries. FIG. 8 illustrates the implementation of present embodiment in fabrication of graded structured solid oxide fuel cell unit using DED with multi material reservoirs. The DED process is initiated when a laser beam is directed onto a starting substrate material. Laser beam characteristics, such as power, beam size, and distance from the substrate are controlled in a manner that optimizes the desired build. The laser energy imparted to the substrate creates a liquid melt pool into which new materials are added. Materials are added by inducing flow of such materials from storage hoppers, through a connecting feed tube, into the melt pool. The laser beam position and corresponding feed tube position are continuously moved in a controlled manner to form the desired geometry. New material that that flows through the feed tube originates from storage hoppers. Multiple storage hoppers, each of which can be filled with different materials, allow for parts to be built with different materials and gradations of materials. As shown in FIG. 9 , three powder hoppers are utilized: one for YSZ electrolyte, one for NiO anode, and one for lanthanum strontium manganite (LSM) cathode. Flow rate of powder material from each hopper to the fed tube is controlled in this process. Precisely controlled powder mix with desired ratio is fed into tube and further to powder spray nozzle. In FIG. 8 , the depiction of the laser forming the melt pool on a substrate while powdered material is impinged upon the melt pool shows the process of DED in action as a multi-material graded structure is deposited on a substrate.

The fabricated components that are fabricated based on the present embodiment are characterized. Micrographs of three major components in solid oxide fuel cells are illustrated in FIG. 9 . The cathode fabricated from LSM powder, electrolyte from YSZ powder and anode from NiO powder shows continuous and solid microstructure as shown in FIG. 9 . No pores or voids are observed in the micrographs.

FIGS. 10A, 10B, and 10C show pictures of 2.5 cm*2.5 cm unit with 100 μm thick YSZ on 200 μm thick NiO. The interface between YSZ and NiO are graded structure with gradient transition on chemical composition. Pictures show the continuous and solid surface of electrolyte YSZ. Micrographs also show continuous microstructure in micrometer scale. The graded structure at interface for solid oxide fuel cell components has been reached successfully by DED.

The depiction in FIG. 10A shows micrographs of the cathode of the fuel cell, FIG. 10B shows a micrograph of the electrolyte of the fuel cell, and FIG. 10C shows a micrograph of the anode of the fuel cell. The cathode, electrolyte, and the anode have all been produced using a multi-material graded structure. The material has been deposited using a DED process where powdered material is formed into a melt pool by a high energy source such as a laser and deposited on a substrate.

The depiction in FIG. 11A shows a fuel cell portion manufactured with additive manufacturing while FIG. 11B depicts a magnified portion of the multi-material graded structure in FIG. 11A for more detail of the deposition of the powdered material that has been subsequently melted in the melt pool and applied to the substrate.

The depiction of the fuel cell in FIG. 12 shows the separation of the cathode, the electrolyte, and the anode. It also shows the electrochemical cycle where fuel is put into the fuel-cell and heat, electricity, and water are products of the reaction. This depiction shows distinct layers of the anode electrolyte cathode, unlike the multi-material graded structure that is formed by the additive manufacturing process utilizing the DED process and powdered materials the form a melt pool which is subsequently deposited on a substrate.

The multi-material graded structure that is shown in FIG. 13 depicts the interface between the electrolyte anode layers of the fuel-cell. It may be seen that the interface is made up of a multi-material graded structure that changes gradually from one layer to the next.

FIG. 14A shows a reactor 700 with complex channeled structure, which is not manufacturable by traditional manufacturing method. FIG. 14B is a cut away of the additive manufactured structure in FIG. 14A showing the interior channels. The only feasible method to fabricate it is through additive manufacturing. Using additive manufacturing methods with multi-material capability, it is possible to fabricate the complex structure with both a composition gradient and a structure gradient. The internal structure 702 may be curvilinear or other complex shapes while the exterior 701 may be a plain in shape three dimensional polygon such as a cylinder or cube.

A solid oxide fuel cell has two categories: anode support or cathode support. The size of the unit cell is in the range of 4-2500 cm² with side 5-50 cm. The thickness of the electrolyte is between 3-300 μm. The gradient change from electrode to electrolyte is between 10 μm⁻¹-10 mm⁻¹. In other words, 20% composition change within 4-80 μm. Several different embodiments for SOFC structures can be considered, depending upon desired geometry (planar, tubular, monolithic, unique geometry) of final product. In one tubular embodiment, the cathode is relatively thick compared to the electrolyte, anode, and interconnect. For this case, a graded structure may be envisioned as shown in FIG. 1 . The grading between cathode and electrolyte may be uniform at 10% grading per 200 μm thickness. Similarly, the anode in such embodiment may be graded at a higher level, at 10% grading per 15 μm thickness.

The defined range of controlled additive manufacturing method process parameters that result in quality builds of graded of graded structures of any geometric shape. Control and definition of the additive manufacturing process parameters dictates the quality of the continuously built parts. The defined and/or controlled parameters during continuous build include: type of laser, laser power, laser wavelength, laser spot size, laser focal point, laser beam profile, melt pool size, total material flow rate through feed tube, flow rate of each material from each powder hopper, write speed, individual layer dimensions, distance from laser head to part, and distance from powder feed tube to part.

In some cases, the defined and/or controlled parameters during continuous build include:

The type of laser technologies including gas laser (such as CO₂ laser), chemical laser (such as hydrogen fluoride laser), solid state laser (such as ytterbium doped glass fiber laser), dye laser (such as coumarin 102 laser), metal-vapor laser (such as helium cadmium laser). In some cases, laser power for additive manufacturing spans the range from less than 20 watts up to 20,000 watts in constant power or pulsed modes. Laser wavelengths typically ranging from 0.193 μm to 10.6 μm, depending on type of laser. Laser spot size depends on laser system and can be less than 28 μm and up to and exceeding 500 μm. Laser focal point can be positioned at converging or diverging sections of the laser beam. The laser beam profile can be “top hat” or Gaussian, or another suitable profile. The laser beam quality factor is typically between 0.3-20 mm*mrad, although other quality factors may be used.

In embodiments, the melt pool size is controlled by a combination of laser type, wavelength, power, beam size, write speed, material flow (in the case for directed energy deposition). Total material flow rate through the feed tube typically has a typical range of about 0.5-5 grams/minute depending upon powder density. The flow rate of each material from each powder hopper is proportional to the number of hoppers scaled to the total material flow rate through the feed tube. In some cases, the write speed of the DED process is between 0.5-3.0 cm/sec.

The individual layer dimensions range from less than 20 μm to more than 150 μm, or about 20 μm to about 150 μm, or about 30 μm to about 100 μm depending upon powder particle size, laser power size and laser spot size.

The distance from laser head to part typically ranges from 1 cm to up to 10 cm, or about 3 cm to about 8 cm, while the distance from powder feed tube to part typically ranges from 1 cm to up to 10 cm, or about 3 cm to about 8 cm.

In embodiments, controlled and defined parameters for additive manufacturing methods disclosed herein for continuous fabrication of components and systems will impart the following specific properties and characteristics to the built parts:

geometric dimensions, material adhesion between each layer, porosity, composition, including gradients, and mechanical and chemical properties.

A number of alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims. 

We claim:
 1. A method for producing a multi-material graded structure wherein an additive manufacturing process utilizing a high energy source and multiple material storage is used to produce a melt pool of multi-materials that is formed from materials fed into the melt pool at a precise and predetermined rate and mixture, and wherein the multi-material from the melt pool is deposited upon a substrate to form a solid graded multi-material comprising at least one of a solid cathode, a solid anode and a solid electrolyte.
 2. The method of claim 1 wherein the solid graded multi-material comprises the solid cathode, the solid anode, and the solid electrolyte.
 3. The method of claim 2 wherein the electrolyte comprises yttria-stabilized zirconia (YSZ).
 4. The method of claim 2 wherein the cathode comprises lanthanum strontium manganite (LSM).
 5. The method of claim 2 wherein the electrolyte comprises yttria-stabilized zirconia (YSZ), the anode comprises NiO, and the cathode comprises lanthanum strontium manganite (LSM).
 6. The method of claim 1 wherein the additive manufacturing process includes powder bed additive manufacturing.
 7. The method of claim 1 wherein the high energy source comprises a laser source.
 8. The method of claim 1 wherein the multi-material graded structure is configured to be included in a solid oxide fuel cell.
 9. The method of claim 1 where the multi-material graded structure is a solid oxide fuel cell.
 10. The method of claim 1 wherein the multi-material graded structure is included in a battery.
 11. The method of claim 10 wherein the battery includes a positive grid, a positive plate, a separator, a negative grid and a negative plate, at least one of which comprises the multi-material graded structure.
 12. The method of claim 1, wherein the multi-material graded structure is graded in porosity.
 13. The method of claim 1, wherein the multi-material graded structure is graded in chemical composition.
 14. The method of claim 1 wherein the high energy source comprises an electron beam source.
 15. A method for producing a multi-material graded structure wherein an additive manufacturing process utilizing a high energy source and multiple material storage is used to produce a melt pool of multi-materials that is formed from materials fed into the melt pool at a precise and predetermined rate and mixture, and wherein the multi-material from the melt pool is deposited upon a substrate to form a solid graded multi-material comprising at least one of an energy storage unit, an energy conversion unit, and a chemical reformer.
 16. The method of claim 15 wherein the high energy source comprises a laser source.
 17. The method of claim 15 wherein the multi-material graded structure is graded in porosity.
 18. The method of claim 15 wherein the multi-material graded structure is graded in chemical composition.
 19. A method for producing a multi-material graded structure wherein an additive manufacturing process utilizing a high energy source and multiple material storage is used to produce a melt pool of multi-materials that is formed from materials fed into the melt pool at a precise and predetermined rate and mixture, and wherein the multi-material from the melt pool is deposited upon a substrate to form a solid graded multi-material that is included in a battery.
 20. The method of claim 19 wherein the battery includes a positive grid, a positive plate, a separator, a negative grid and a negative plate, at least one of which comprises the multi-material graded structure. 